Photochromic substances exhibit a reversible change when exposed to light radiation involving ultraviolet rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp. Generally, photochromic substances contain acrylate monomers and photochromic dyes, in which acrylate monomers are crosslinked by UV to form a photochromic coating film.
Photochromic dyes such as spirooxazine or naphthopyran series compounds are generally used. Photochromic dyes exhibits color change depending on conformational change such as ring closing or opening reaction, which is contained in the compound. Energy source needed for conformational change of the molecules is typically ultraviolet light at a wavelength of 360 to 380 nm, even though there is a difference between specific substances.
On the other hand, to cure the photochromic compositions containing acrylate monomers, very strong ultraviolet is required. Thus, the photochromic dye contained in the photochromic composition is degraded to reduce the optical density. Ultimately, suitable photochromism is not ensured to problematically generate reduction in durability of the formed coating film.
Meanwhile, in the case of using a thermally curable urethane-based resin, such problems generated in the photo-curable photochromic coating film can be avoided, but too much time is needed for curing. In order to solve the above-mentioned problems, many efforts have been made, thereby leading to the present invention.